A common type of camera is shown in FIG. 1. An area (2) of a body (4) of a patient containing a radioactive pharmaceutical, emits gamma rays indicated as at (6). The camera (8) includes a collimator comprising an apertured lead sheet (10) so that only gamma rays within a predetermined narrow angle from the patient can pass through the collimator to a scintillation block (12) of Nal(T1). Single gamma rays entering block (12) give rise to a large number of secondary photons which radiate outwards through a glass light guide and support block (14) to an array of photomultipliers (16). Photomultipliers (16) are usually arranged in a rectangular or hexagonal grid. Each photomultiplier tube is arranged to detect individual photons or groups of photons to produce a significant electrical signal which is conducted to a signal processing unit (18). Unit (18) assesses all the signals received from the individual photomultiplier tubes to carry out an analysis of the signals received at consecutive time instants in order to determine approximately where each gamma ray impinges on scintillation block (12).
Position reconstruction in a gamma camera is usually performed by taking a linear weighted mean of the signals of the photomultiplier array, see for example US-A-4228315. Problems arise with signals from photomultipliers far from a gamma ray event where only a very small number of photons are detected. Since the statistical noise fluctuations are proportional to the square root of number of photons detected, the fluctuation in small signals is comparable to the signal itself. Because of this relatively large statistical noise fluctuation some form of limiting has to be applied to the smallest signals to exclude noise in order to obtain better spatial resolution. This may be done with a thresholding device, for example a diode, which provides a fixed thresholding level and which therefore only passes signals from a photomultiplier tube having a voltage level greater than the junction voltage of the diode. Whilst this provides a simple way of excluding low value signals which have a large amount of noise associated therewith, some useful information is also discarded. Now each gamma ray incident will create a distinctive energy dependent distribution curve of detected photons. Therefore, if a fixed threshold level is subtracted from each photomultiplier tube signal, the resulting distribution curve is effectively distorted during measurement and position reconstruction becomes inaccurate.
Referring now to FIG. 3, this shows a hexagonal array of close-packed photomultiplier tubes; this is a preferred configuration from the point of view of signal processing since the nearest neighbors of each tube are spaced the same distance R from the center of the tube as indicated in FIG. 3. This simplifies analog processing of the signals. However a problem arises with the hexagonal array in that at the edges of the array there are gaps as indicated in FIG. 3 in the region (30), which give rise to a reduced field of view indicated by the dotted line (32). This is a particular problem with arrangements such as shown in FIG. 4 wherein two cameras (40) are arranged at right-angles, contacting one another at their edges in order to get an improved view of a side region (42) of a patient's torso (44), for example the head. The result of the reduced field of view in the corner area is to create a dead region (46) for detection, with the result that the two cameras have to be positioned relatively far from the patient, resulting in a loss of resolution.
A preferred configuration which avoids the reduced field of view is to employ square photomultiplier tubes (20) arranged in a rectangular grid as shown in FIG. 2. In this arrangement the array extends right up to the edge of the camera. However, a disadvantage is that the nearest neighbors of each tube are spaced at various distances from the center of the tube as indicated by the dimensions R1 and R2 in FIG. 2. This places greater demands on analog signal processing required to identify the spatial location of a gamma ray incident. With a rectangular array therefore the noise effects and threshold effects are likely to be more pronounced. These effects will be exacerbated when large aperture photomultiplier tubes are employed for low cost. Clearly smaller photomultiplier tubes increase the overall resolution of the array, but larger tubes are preferred from the point of view of expense.